My solution for storing sandpaper for my various sanders.

Inspired by something that Kyle told me about seeing on the Internet and constantly fearful I’ve left the house with the garage door open I made an automatic door closer this evening.

I walked into Active Electronics with a schematic more or less like this. They must have thought I was an idiot but they helped me pick out the right relay non-the-less.

The basic idea is to piggy back off the garage opener light. When the garage opener opens the door it turns on the light for about 5 minutes. I just hook into that and try and close the door when the light goes out.

The hardest part was the door switch. I only want my circuit to work when the door is up. I don’t want to have it open up the door when closed by accident. I’m still not particularly happy with what I created here but it seems to work. I made to coat hanger probes that bump into a metal peace on the door when the door is up all the way. When the door goes down the circuit is open.

Here is the inside of the opener. I just tucked the relay inside and hooked with wires into the outlet for the lightbulb.

It seems that by the end of the day I end up with more tabs than are useful in Chrome.

For an upcoming meeting I bought 20 pairs of red-cyan 3D glasses from Deal Extreme (these ones here).  I figured since 3D is the big thing in the theatres it should be in our sprint reviews too.

There is an excellent free application called Anaglyph Maker that can combine two images into a single red-cyan 3D anaglyph.  Hand holding the camera and moving it over about 4” between shots ended up with a surprisingly descent 3D image.

I started running as an excuse to play with the nifty Nike+ stuff built into my iPod.  It turns out I really quite like the running.  It also turns out that every time I look at the Nike+ website I’m offended by how bad it is. 

It’s completely flash based and effectively prevents me from getting at any of my raw data.  That annoys me but if the site itself was good that wouldn’t really be a big deal.

The problem is that the site does a horrendous job of representing data.  The above graph is typical.  Here we can see that I started off slow, got faster, got slower, got faster, etc.  How fast?  How slow?  Well you aren’t going to find out because apparently the good people at Nike didn’t pay attention in science class where you were told how to create graphs with correctly labeled axis.  Distance is a little better because you can  hold your mouse over the somewhat arbitrary white circles and it will give the distance of that circle.

It’s such a neat gadget and has the potential to be so cool.  It’s a shame they produced such a lack luster front-end for it..

I’m a little late switching over but I’m really starting to like Chrome.  It’s just so fast!

I generally leave a browser open for days on end with a number of sites (gmail, google cal, facebook, etc) open all the time.  These make for a very busy tab strip.

Today I was thrilled to find that if you right-click on a tab you can pin it so that it moves over the the left and shrinks to just the favicon.

I wish there were a nice way to persist my pinned tabs but I’ll take it.

The Daily WTF posted this neat Key Lock Box problem.

This was actually the first time I had ever seen a key lock box. Essentially, they’re small steel boxes that contain a key and are often affixed near a keyhole (eg., the door handle). Some feature a set of dials that must be aligned with the right code, while others (such as the one pictured) use a pushbutton system to enter the combination.

Being that they’re such low-tech contraptions, the pushbutton key lock boxes don’t quite work like you might expect. For example, one popular brand will unlock if the four-digit code is entered in sequence, regardless of what or how many digits preceded it. That is, if the code is 2-3-4-5, pressing 1-2-3-4-5 will open the box. Taking it a step further, the sequence 4-8-2-9-5-1-4-5 would cover the codes 4-8-2-9 8-2-9-5 9-5-1-4 and 5-1-4-5.

Your exercise for the day: write a function that outputs a sequence numbers which cover all combinations within the sequence.

Input: two variables: an integer that represents the combination code length, and the number of buttons on Output: The output should be a valid sequence that covers all combinations

My solution is the 10,124 character long string:

000000010002000300040005000600070008000900110012001300140015
001600170018001900210022002300240025002600270028002900310032
003300340035003600370038003900410042004300440045004600470048
004900510052005300540055005600570058005900610062006300640065
006600670068006900710072007300740075007600770078007900810082
008300840085008600870088008900910092009300940095009600970098
009901010102010301040105010601070108010901110112011301140115
011601170118011901210122012301240125012601270128012901310132
013301340135013601370138013901410142014301440145014601470148
014901510152015301540155015601570158015901610162016301640165
016601670168016901710172017301740175017601770178017901810182
018301840185018601870188018901910192019301940195019601970198
019902020203020402050206020702080209021102120213021402150216
021702180219022102220223022402250226022702280229023102320233
023402350236023702380239024102420243024402450246024702480249
025102520253025402550256025702580259026102620263026402650266
026702680269027102720273027402750276027702780279028102820283
028402850286028702880289029102920293029402950296029702980299
030303040305030603070308030903110312031303140315031603170318
031903210322032303240325032603270328032903310332033303340335
033603370338033903410342034303440345034603470348034903510352
035303540355035603570358035903610362036303640365036603670368
036903710372037303740375037603770378037903810382038303840385
038603870388038903910392039303940395039603970398039904040405
040604070408040904110412041304140415041604170418041904210422
042304240425042604270428042904310432043304340435043604370438
043904410442044304440445044604470448044904510452045304540455
045604570458045904610462046304640465046604670468046904710472
047304740475047604770478047904810482048304840485048604870488
048904910492049304940495049604970498049905050506050705080509
051105120513051405150516051705180519052105220523052405250526
052705280529053105320533053405350536053705380539054105420543
054405450546054705480549055105520553055405550556055705580559
056105620563056405650566056705680569057105720573057405750576
057705780579058105820583058405850586058705880589059105920593
059405950596059705980599060606070608060906110612061306140615
061606170618061906210622062306240625062606270628062906310632
063306340635063606370638063906410642064306440645064606470648
064906510652065306540655065606570658065906610662066306640665
066606670668066906710672067306740675067606770678067906810682
068306840685068606870688068906910692069306940695069606970698
069907070708070907110712071307140715071607170718071907210722
072307240725072607270728072907310732073307340735073607370738
073907410742074307440745074607470748074907510752075307540755
075607570758075907610762076307640765076607670768076907710772
077307740775077607770778077907810782078307840785078607870788
078907910792079307940795079607970798079908080809081108120813
081408150816081708180819082108220823082408250826082708280829
083108320833083408350836083708380839084108420843084408450846
084708480849085108520853085408550856085708580859086108620863
086408650866086708680869087108720873087408750876087708780879
088108820883088408850886088708880889089108920893089408950896
089708980899090909110912091309140915091609170918091909210922
092309240925092609270928092909310932093309340935093609370938
093909410942094309440945094609470948094909510952095309540955
095609570958095909610962096309640965096609670968096909710972
097309740975097609770978097909810982098309840985098609870988
098909910992099309940995099609970998099911111112111311141115
111611171118111911221123112411251126112711281129113211331134
113511361137113811391142114311441145114611471148114911521153
115411551156115711581159116211631164116511661167116811691172
117311741175117611771178117911821183118411851186118711881189
119211931194119511961197119811991212121312141215121612171218
121912221223122412251226122712281229123212331234123512361237
123812391242124312441245124612471248124912521253125412551256
125712581259126212631264126512661267126812691272127312741275
127612771278127912821283128412851286128712881289129212931294
129512961297129812991313131413151316131713181319132213231324
132513261327132813291332133313341335133613371338133913421343
134413451346134713481349135213531354135513561357135813591362
136313641365136613671368136913721373137413751376137713781379
138213831384138513861387138813891392139313941395139613971398
139914141415141614171418141914221423142414251426142714281429
143214331434143514361437143814391442144314441445144614471448
144914521453145414551456145714581459146214631464146514661467
146814691472147314741475147614771478147914821483148414851486
148714881489149214931494149514961497149814991515151615171518
151915221523152415251526152715281529153215331534153515361537
153815391542154315441545154615471548154915521553155415551556
155715581559156215631564156515661567156815691572157315741575
157615771578157915821583158415851586158715881589159215931594
159515961597159815991616161716181619162216231624162516261627
162816291632163316341635163616371638163916421643164416451646
164716481649165216531654165516561657165816591662166316641665
166616671668166916721673167416751676167716781679168216831684
168516861687168816891692169316941695169616971698169917171718
171917221723172417251726172717281729173217331734173517361737
173817391742174317441745174617471748174917521753175417551756
175717581759176217631764176517661767176817691772177317741775
177617771778177917821783178417851786178717881789179217931794
179517961797179817991818181918221823182418251826182718281829
183218331834183518361837183818391842184318441845184618471848
184918521853185418551856185718581859186218631864186518661867
186818691872187318741875187618771878187918821883188418851886
188718881889189218931894189518961897189818991919192219231924
192519261927192819291932193319341935193619371938193919421943
194419451946194719481949195219531954195519561957195819591962
196319641965196619671968196919721973197419751976197719781979
198219831984198519861987198819891992199319941995199619971998
199922222223222422252226222722282229223322342235223622372238
223922432244224522462247224822492253225422552256225722582259
226322642265226622672268226922732274227522762277227822792283
228422852286228722882289229322942295229622972298229923232324
232523262327232823292333233423352336233723382339234323442345
234623472348234923532354235523562357235823592363236423652366
236723682369237323742375237623772378237923832384238523862387
238823892393239423952396239723982399242424252426242724282429
243324342435243624372438243924432444244524462447244824492453
245424552456245724582459246324642465246624672468246924732474
247524762477247824792483248424852486248724882489249324942495
249624972498249925252526252725282529253325342535253625372538
253925432544254525462547254825492553255425552556255725582559
256325642565256625672568256925732574257525762577257825792583
258425852586258725882589259325942595259625972598259926262627
262826292633263426352636263726382639264326442645264626472648
264926532654265526562657265826592663266426652666266726682669
267326742675267626772678267926832684268526862687268826892693
269426952696269726982699272727282729273327342735273627372738
273927432744274527462747274827492753275427552756275727582759
276327642765276627672768276927732774277527762777277827792783
278427852786278727882789279327942795279627972798279928282829
283328342835283628372838283928432844284528462847284828492853
285428552856285728582859286328642865286628672868286928732874
287528762877287828792883288428852886288728882889289328942895
289628972898289929292933293429352936293729382939294329442945
294629472948294929532954295529562957295829592963296429652966
296729682969297329742975297629772978297929832984298529862987
298829892993299429952996299729982999333333343335333633373338
333933443345334633473348334933543355335633573358335933643365
336633673368336933743375337633773378337933843385338633873388
338933943395339633973398339934343435343634373438343934443445
344634473448344934543455345634573458345934643465346634673468
346934743475347634773478347934843485348634873488348934943495
349634973498349935353536353735383539354435453546354735483549
355435553556355735583559356435653566356735683569357435753576
357735783579358435853586358735883589359435953596359735983599
363636373638363936443645364636473648364936543655365636573658
365936643665366636673668366936743675367636773678367936843685
368636873688368936943695369636973698369937373738373937443745
374637473748374937543755375637573758375937643765376637673768
376937743775377637773778377937843785378637873788378937943795
379637973798379938383839384438453846384738483849385438553856
385738583859386438653866386738683869387438753876387738783879
388438853886388738883889389438953896389738983899393939443945
394639473948394939543955395639573958395939643965396639673968
396939743975397639773978397939843985398639873988398939943995
399639973998399944444445444644474448444944554456445744584459
446544664467446844694475447644774478447944854486448744884489
449544964497449844994545454645474548454945554556455745584559
456545664567456845694575457645774578457945854586458745884589
459545964597459845994646464746484649465546564657465846594665
466646674668466946754676467746784679468546864687468846894695
469646974698469947474748474947554756475747584759476547664767
476847694775477647774778477947854786478747884789479547964797
479847994848484948554856485748584859486548664867486848694875
487648774878487948854886488748884889489548964897489848994949
495549564957495849594965496649674968496949754976497749784979
498549864987498849894995499649974998499955555556555755585559
556655675568556955765577557855795586558755885589559655975598
559956565657565856595666566756685669567656775678567956865687
568856895696569756985699575757585759576657675768576957765777
577857795786578757885789579657975798579958585859586658675868
586958765877587858795886588758885889589658975898589959595966
596759685969597659775978597959865987598859895996599759985999
666666676668666966776678667966876688668966976698669967676768
676967776778677967876788678967976798679968686869687768786879
688768886889689768986899696969776978697969876988698969976998
699977777778777977887789779877997878787978887889789878997979
79887989799879998888888988988899898989999000

I have the codes in 8889 and 9888 in there twice so it’s a tad longer than it needs to be. I think the optimal solution is the De Bruijn Sequence but that involves math I don’t understand.

My solution was written in Python.

def permutations(values, size):
    if size == 0: return [[]]
    out = []
    for v in values:
        for perm in permutations(values, size-1):
            out.append([v] + perm)
    return out

def combinationString(permutations):
    out = ''
    for permutation in permutations:
        tmp = ''.join([str(num) for num in permutation])
        if out.find(tmp) == -1:
            out += tmp
    return out;

def verify(permutations, combinationString):
    for permutation in permutations:
        tmp = ''.join([str(num) for num in permutation])
        m = combinationString.split(tmp)
        if len(m) == 1: print ("[ERROR]", tmp, "is missing")
        if len(m) > 2: print ("[WARN]", tmp, "is duplicated")

v = [0,1,2,3,4,5,6,7,8,9]
s = 4
p = permutations(v, s)
c = combinationString(p)
verify(p, c)

print("Combination String:", c)
print(len(c))

Today my work computer started behaving badly…

and SQL Server Management Console…

Nice error… “The file exists.”  Fortunately the details were actually helpful…

Calls to “GetTempFileName “ are failing and the word on the Intertubes is that will happen if you end up with > 65,000 files in your temp folder.

Which, apparently, I have accumulated.  Cleaning those out and everything works wonderfully again.

FAIL to all the developers of the software I use that are leaving all these files here…

We’re working on redesigning our licensing code and part of that involved generating a unique installation ID for each installation of our software.  The goal is for this identifier to be absolutely unique for each client while still being small enough that it would be feasible for them to read it over the phone to us. 

The identifier is generated from hashing together a lot of system data and converting that into a N digit code, where N is large enough to be unique but small enough not to be cumbersome.

That got us thinking…

For various identifier lengths, how many identifiers could we generate before we had, say, a 1% chance of a collision?

This is an implementation of the birthday paradox (ie. what are the chances two students in a class of 30 share the same birthday). 

It turns out that the math to solve this problem produces some absolutely ginormous numbers.  Excel gave up rather quickly as did Python’s built-in numeric types but fortunately the add-on mpmath library can handle such ridiculous numbers with ease.

A lunch hour of poking at it yielded the following chart.

From this you can see that with a key size of only 10 digits and only a few thousand identifiers you are risking a collision while for  a 12 digit key one can safely handle 100,000 + identifiers.  Moving to 15 or 20 digit identifiers makes the chance of collision insignificant unless we are dealing with enormous numbers of identifiers (which we are not).

I’ve created an online calculator for the birthday paradox here.

from mpmath import *
def collisionProbability(nKeys, nEntities):
    t1 = mpf(1)
    for i in range(nKeys-nEntities+1, nKeys+1):
        t1 *= i
    return 1 - t1 * power(mpf(1)/nKeys,nEntities)